Crack open the cranium of a London cab driver and you're liable to find that they're packing extra grey matter in regions of the brain responsible for spatial navigation. But this observation poses a compelling question: are the street's most brilliant cabbies made, or are they born?

To find out, researchers Katherine Woollett and Eleanor Maguire studied the brains of drivers-in-training as they acquired what London cabbies call "the Knowledge" — the Gordian layout of 25,000 streets and 20,000 landmarks within a 6-mile radius of the city's bustling Charing Cross train station (the map featured below shows just a portion of the total area that London's cab drivers are responsible for being intimately familiar with).

A typical trainee takes 3-4 years to acquire "the Knowledge," which they must draw from in order to complete the grueling examinations required to obtain an operating license from the city's official taxi-licensing body. This qualification process is unlike any other in the world, and offers the unique opportunity to conduct long-term evaluations on the brains of cabbie hopefuls.

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At the beginning of the study, Woollett and Maguire selected 79 trainees and 31 non-taxi-driver controls. Test participants were assessed on factors such as IQ, memory skills, and grey matter volume — but no significant differences in general intellect, powers of recollection, or brain structure could be identified between the groups.

Three to four years later, however, the results looked very different. Of the 79 original trainees, 39 went on to qualify as licensed London taxi drivers, and 20 did not (of the 20 trainees who did not return for a second round of testing by Woollett & Maguire, only 2 had managed to qualify). The researchers found that trainees who had qualified had experienced an increase in gray matter in the back part of the hippocampus, a region of the brain which plays a crucial role in memory and spatial navigation (this feature had also been observed in the brains of cab drivers in prior cross-sectional studies). Neither the drivers who had failed to qualify, nor the non-trainee controls, experienced such boosts in brain matter.

So does this close the book on the made-not-born debate? Not exactly. The researchers have no away of knowing whether those drivers who managed to qualify had some inborn advantage over those who didn't. "Could it be that those who qualified are genetically predisposed towards having a more adaptable, 'plastic' hippocampus?" asks Maguire. "This leaves the perennial question of 'nature versus nurture' still open."

What the team's findings do support, Maguire says, is that the human brain remains plastic well into adult life, allowing it to adapt when we learn to perform new tasks — something the researchers say should come as encouraging news for life-long learning and rehabilitation following brain injury.

The researchers' findings are published in the latest issue of Current BiologyTop image via; London map via Current Biology